"I think our study highlights the fact that we still have a long
way to go until we fully understand the extinction of the
dinosaurs," lead author Stephen Brusatte, a Columbia University
graduate student affiliated with the American Museum of Natural
History's Division of Paleontology, told Discovery News. The
study was published in the latest issue of Nature Communications.

"There are a couple of things we know for sure," he added. "We
know a large asteroid or comet hit the planet about 65.5 million
years ago, right when the dinosaurs completely disappeared from
the fossil record."

"We also know there was massive volcanism and major sea level
changes at this time. We now also know that at least some groups
of dinosaurs were undergoing long-term declines in biodiversity
during the final 12 million years of the Cretaceous, at least in
North America."

The study presents the first look at dinosaur extinction based on
morphological disparity, meaning the variability of body
structure within particular groups of dinosaurs. The more the
variability in a species, generally, the healthier the population
was.

Earlier research was based almost always on estimates of change
in the number of dinosaur species over time, but that can be
affected by uneven sampling within the fossil record.

Some geological formations, for example, tend to preserve
dinosaur remains better than others.

Brusatte and his team calculated differences in body size for
seven major dinosaur groups using databases that include
wide-ranging characteristics about the intricate skeletal
structure of nearly 150 different species.

They discovered that large-bodied, bulk-feeding plant-eaters were
dying out long before the natural disasters of 65.5 million years
ago. These animals included hadrosaurs and ceratopsids.

On the other hand, small plant-eaters (ankylosaurs and
pachycephalosaurs), carnivorous dinosaurs (tyrannosaurs and
coelurosaurs) and huge plant-eaters without advanced chewing
abilities (sauropods) remained fairly stable over the same period
of time.

While the die-off of the larger species remains a mystery,
Brusatte said, "Something was going on with large herbivores in
the late Cretaceous, at least in North America. Maybe it was the
fact that the local environments were in flux due to drastic sea
level changes and mountain building at the time."

He explained that plant-eaters may have felt the effects of a
changing land area first since they sat at the bottom of the food
chain.

"Maybe, given a few more million years we would have seen
declines in other dinosaur groups higher up in the food chain,"
he said.

Paul Upchurch, a University College London paleobiologist,
doesn't buy it and stands by the idea that a big asteroid wiped
out the dinosaurs.

"First, only some dinosaur groups show reduced disparity in the
final 12 million years, while other groups continue to do well.
So this study could actually be taken as evidence in favor of a
sudden extinction," Upchurch added. "We need a mechanism that
explains why the smaller dinosaurs and large sauropods died out
suddenly at the end of the Cretaceous."

Second, he argues that a more extensive look at all dinosaur
history is needed to see if such population declines happened
more than once over the 165 million years that dinosaurs were in
existence.

"The decline in disparity during the final 12 million years might
merely be 'evolutionary business as usual' and have little to do
with the true final extinction," he said.

Brusatte agrees that his team's findings are debatable, "but at
the very least we can't envision the latest Cretaceous as a
static, idyllic lost world that was suddenly exterminated by an
asteroid impact.

"Instead, the dinosaurs living during this time were undergoing
major changes before the asteroid hit."